Sam Katzl Naval Medical Research institute Bethesda, Maryland
A Simple Thermoregulator
A
heptane-mercury thermoregulator capable of maiutaining temperature constant within 0.002'C between the range of -20' to 50°C is described. It differs from other regulators of similar sensitivity by its simplicity of fabrication, sturdy construction, and ease of filling. It can be made by any individual possessing a minimal proficiency in glassblowing. The details of construction are presented in the accompanying figure. The dimensions are not critical, being determined by the geometry of the thermostat..
'The opinions or assertations contained herein are the private ones of the writer and are not to be construed as official or reflecting the views of the Navy Department or the naval service at lwgr?.
A platinum electrode is sealed in one limb so that it is immersed in mercury, while the adjustable stainless steel l e d , ? the fine control, is seated in the capillary. The inner diameter of the thick-walled glass capillary is 0.5 mm. The unit is charged by initially evacuating the system by applying a vacuum via the capillary with the Teflon stopcock closed. Then the tube leading to the vacunm source is sealed, and heptane is added through the stopcock until it is about 75% filled. From 10 to 30 ml of mercury is put in a layer on the bottom, followed by additional heptane until the solvent reaches the position indicated in the diagram. The final temperature setting is made by adding mercury and adjusting the fine variable lead in the conventional manner. The sensitivity of a thermoregulator is a function of the volume change produced by a change of temperature and by the rate of heat transfer from the thermostat to the sensing unit and vice versa. This unit capitalizes on the first factor, i.e., employing a large volume, 300 ml, of heptane-a material characterized by a large coefficient of cubical expansion, 1.22 X 10-3/degree.3 Consequently a temperature change of O.OOl°C will produce a volume change of 0.36 microliters or a change of 1.85 mm in the capillary. Obviously, the thermostatic element would be activated by a smaller temperature change. However, the primary factor determining the sensitivity of a thermoregulator is the rate of heat transfer. The rate expression for heat transfer due to conduction is as follows:
where 6 is the rate of heat flow across a surface element of area, A ; under a temperature gradient, dT/dX where K is the thermal conductivity. Inspection of this equation will reveal that an ideal thermoregulator should expose the maximum possible surface relative to volume and should incorporate materials with high coefficients of expansion, thermal conductivity and thermal diffusivity. I t is the last two factors which are the basis for the ~idespreaduse of mercury-since it has excellent thermal and electrical conductivity properties. Yet the relatively low coefficient of expansion, 0.18 X lo-=/ degree, and the high density of mercury impose serious restrictions upon instrument design.
A diasrommotis representation of the heptane-mercury thermoregulator.
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Journal of Chernicol Education
It is essential that the mercurysurface be clean, but it ismole important still that the mercury should not be wet and adhere to the contact element. The use of "tinned" copper wire is to be avoided for this reason. ""anddt-BarnsteinPhysikalisrh-CbemiseheTabl" edited ~ ~ R o T W. H ,A,, and SCHEEL,K., 5th Auflage,2 Band, J. Springer, Berlin, 1923, p. 1224.
To circumvent the use of mercury, one category of thermoregulators employs materials with high coefficients of expansion, such as toluene or heptane. But since these substances are characterized by low thermal cond~ct~ivity, the apparatus must he designed to maximize the surface area relative to the volume in order to compensate for their poor conductivity. The resultant unit is generally fragile and susceptible to breakage. In the unit described here, both mercury and heptane are used. Since t,he density of heptane is low, 0.682 g/ml, the use of thin glass in the circular section of this regulat,or is permitted, eliminating the thermal lag attributable to glass, a poor thermal conductor. Heat transfer is further facilitated by the mercury on the bottom of the reservoir, due to its excellent thermal conductivity. In addition to the heat transfer affected by conductivity there is some transfer due to convection. The geometry of the circular reservoir promotes t,he convectional movement of heptane arising from any pensity differences resuking from temperature gradients.
The temperature monitored by this regulat,or is an integrated quantity since it encompasses the entire depth of the thermostat. Thus it cannot be unduly influenced by any localized temperat,ure gradient mhich might exist in the bath. The effectivenessof any thermoregulat,or is the function of a thermostat in which it is employed. Adequate stirring is the single most important factor in determining the efficiency of a thermostat. For optimum performance a minimum of 80% of the heat should be furnished by a constant heat source, with the remainder from a low-lag heater controlled by the thermoregulator. For the latter purpose, a GO watt bulb, painted black, connected in series with a Variac can be used. By positioning the intermittent heat source about 5 to 7.5 cm away from a thermoregulator, the tendency for a temperature overshoot to occur is virtually eliminated. The helpful comments of Dr. R. F. Steiner, regarding this apparatus, is acknowledged.
Volume 40, Number 5, May 1963
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